How to inset an extrusion with the rest of the shape?
Start with a cube.

select the cube

press S -&gt; Z and scale it flat as you need

press S -&gt; Shift Z and scale as you need

Tab -&gt; edit mode

Face select




press CTRL - R and insert loop cut



scroll up your mouse wheel until you have 2

click on mouse twice to confirm

select this face




tab E to extrude and extrude as you need



select all top faces



press I and move mouse as you like



now you can extrude again with E if you like



optionally: if you press S and move you mouse you will get:


video tutorial here:
https://youtu.be/mj6LjXgf5c4